1. Field of the Invention
The present invention relates to an electric power assisting steering apparatus, and more particularly, to an electric power assisting steering apparatus which is capable of reducing a noise of a gear.
2. Related Background Art
As an electric power assisting steering apparatus for an automobile, there is known an apparatus which is arranged to decelerate a rotating output of an electric motor acting as an auxiliary steering torque by use of a gear device so as to transmit the decelerated output to an output shaft of a steering mechanism, and to thereby assist a steering effort applied on the steering wheel to steer the automobile. In such an electric power assisting steering apparatus, a power transmitting mechanism provided inside a housing is used to transmit the power to the output shaft, while decelerating the rotation of the electric motor.
In an electric power assisting steering apparatus using, for example, a worm gear mechanism as the power transmitting mechanism, it is necessary to set a proper backlash between the tooth surfaces of the worm and the worm wheel. That is, if such backlash is too small, teeth to mesh with each other tightly so as to make an operational torque heavy, which deteriorates the handle returning.
On the other hand, when the backlash is set to be large to some extent, tight meshing, etc., among the teeth is not caused. Moreover, even when the backlash is set large to some extent, no significant problems will occur if the motive power is transmitted in one direction in the worm gear mechanism. However, in the electric power assisting steering apparatus, the direction of transmission of the power can be changed depending on a steering operation of the steering wheel, a vibration from the road surface via the wheels, or the like.
If the direction of the power transmission is thus changed, a tooth surface on the back of a tooth surface which is so far in contact with a tooth surface of the other tooth suddenly moves by this backlash and collides with another tooth surface of the other tooth, thereby producing a relatively large beating sound. Such beating sound changes depending on the material and the rigidity of gears in mesh, and tends to be larger when the backlash is larger. Especially, when the gears in mesh are made of steel, the beating sound becomes a shock sound which is offensive and unpleasant to the driver.
Such beating sound can be reduced to some extent when one of the worm and the worm wheel is made of resin, but can not be deadened completely. Even in such case, muffled sound of a low frequency may remain.
If the backlash between the tooth surfaces of the worm and the worm wheel is set small, such beating sound can be reduced. However, with the small backlash, the processing accuracy for the worm and the worm wheel must be considerably enhanced so that the costs will be increased, apart from the above-mentioned problems.